A zero cost alternative to gear shift sensors.

Ramon De La Cuetara

This article applies to ebikes equipped with Bafang and similar mid motors. The modification presented has been tested in an ebike equipped with a Bafang BBSHD kit only. It should work with other mid motors that have freewheeling chainrings.

The problem

In a bicycle equipped with multiple sprockets and a derailleur, in order to shift gears the drivetrain must move with very little power applied or a harsh shift will result. This can damage the drivetrain eventually. Since mid motors use the ebike drivetrain to power the rear wheel the motor must be off to shift. If the ebike is using a pedal assist system (PAS), the motor will turn on when pedaling resulting in harsh shifts.

A zero cost solution

We propose the use of a system similar to the front freewheel system (FFS) that Shimano built in the 1970s. The system consisted of a chainring with a freewheel and a rear freewheel with higher friction than normal. The high friction causes the drivetrain to move even when not pedaling so shifting can be done when the bicycle is coasting.

The mid drive ebike already has a chainring freewheel, we only need to modify the rear freewheel to get more friction. The method we use to increase friction is to place an insert between the sprockets and the wheel.

We describe below two methods to increase the friction to the freewheel, using a wedge and using a disk.

Safety Warning
The insert used should allow the freewheel to spin if forced. If by chance an object is trapped between the chain and the chainring the drivetrain will stop, preventing an accident.

Instructions for use

To shift gears:
1) stop pedaling
2) shift
3) resume pedaling after the shit has completed.

Materials (both methods)

A piece of flexible material. The thickness of the material should be slightly larger than the gap between the large sprocket and the chain guard (or the spokes if there is no chain guard). If foam is used it must be of the closed cell type. An open cell will absorb water and may corrode the hub and sprocket.

Wedge method instructions

1. Cut a piece of the flexible material about 1 1/2 inches by 3/4 of an inch ( 40 mm by 10 mm), give it a half moon shape as shown in the photo.

2. Insert the wedge into the gap between the large rear sprocket and the hub as shown, the red arrow points to the wedge. Push it down just below the bottom of the sprocket teeth so it clears the chain.

Disk method instructions

1. Remove the rear wheel from the ebike.

2. Use a freewheel tool to remove the rear freewheel.

3. Cut a disk of the soft material as shown in the photo, the inner diameter should be larger than the hub threads ( or splines in the case of cassette) and the outer diameter should be sufficiently smaller than the largest sprocket so it does not interfere with the chain.

4. Place the disk on the hub as shown.

5. Replace the freewheel. Once tightened try moving the sprockets counterclockwise, they should move with some effort.

6. Replace the wheel